1. Field of the Invention
This invention relates to a thermosetting resin composition, a laminated board employing said composition, and a process for the production thereof.
2. Description of the Prior Art
The laminated materials which have heretofore been used in multilayer printed circuit boards are chiefly laminated boards of phenolic, epoxy and polyimide resins. However, with the recent advance in high speed processing of large-scale computers, there is a demand for printed circuit boards of excellent electric characteristics for the purpose of improving the signal transmission rate. In particular, there is needed a printed circuit board of low permittivity in order to make shorter the signal transmission time-lag and also to decrease the thickness of a board. To meet the need for such low-permittivity laminated materials, there have been developed a laminated polytetrafluoroethylene (PTFE) resin board and a laminated butadiene resin board. The laminated PTFE resin board, however, presents problems such as a large thermal expansion coefficient at elevated temperatures and, unsatisfactory dimensional stability at elevated temperatures because the resin is thermoplastic and of a low glass transition point. Particularly when the laminated PTFE board is used for bonding of, multilayer its reliability of through hole is not sufficient. As a consequence, when it is applied to multilayer printed circuit board its wiring density is taken as in the same level as that of an epoxy resin-base printed circuit board, rendering the merit of low permittivity less significant. Further, since there is no suitable solvent for PTFE, bonding is generally made by applying heat and pressure, though the high melting point is one of the difficulties. In short, as compared with other conventional resins, PTFE presents difficulties in workability and moldability so that large changed must be made in the method of fabrication.
Contrary to the PTFE resin, butadiene resins are thermosetting. However, a butadiene resin which produces a varnish satisfactory for impregnation of the base material and subsequent curing exhibits a disadvantage of residual tackiness in the stage of prepreg, leading frequently to troubles in winding and laminating steps. Moreover, laminated polybutadiene boards are unsatisfactory in peel strength of the copper foil bonded to the board and in other properties such as mechanical strength, flammability, and heat resistance. Although the tackiness can be improved by the incorporation of a higher molecular-weight polymer, the improvement in peel strength of the bonded copper foil and other characteristics of the laminated board require still other means. For this purpose, there is proposed a method in which the polyubutadiene prepreg is overlaid with a coating layer comprising a different type of resin excellent in the properties in question [for example, Japanese Patent Application "Kokai" (Laid-open) No. 74,869/80]. Such a method, however, complicates the operation of lamination and the resulting laminate has a plurality of resin layers having different characteristics, so that properties of the butadiene resins appear not to be fully utilized.